In general, various optical testing systems and methods have been developed for debugging high-performance IC chips. For example, optical testing techniques include Picosecond Imaging for Circuit Analysis (PICA) EMission MIcroscopy (EMMI), Laser Voltage Probe (LVP), Thermal Laser Stimulation (TLS) and Photoelectric Laser Stimulation (PLS). Through optical testing, these optical systems provide non-invasive methods for probing many different points of an IC chip while debugging a problem without necessarily knowing a priori the location of a fault or chip defect. Optical testing allows testing of internal signals of the IC chip provided that the device (transistor) level of the IC chip is optically accessible.
Typically, optical access to the transistors and other components of an IC chip is possible (or practically feasible) only from the back (non-active) side of the IC chip. Indeed, optical testing from the front (active) side surface of the IC chip is significantly impeded by the dense BEOL (back end of line) metallization structures. Most available IC chip packages, however, are designed exclusively for electrical testing and do not take into account the requirements of optical testing from the backside.
One type of IC chip design that can be readily implemented for optical testing from the backside using conventional packaging techniques includes IC chips designed for flip-chip packaging. For example, FIG. 1A schematically illustrates a conventional apparatus (100) for flip-chip packaging. In particular, FIG. 1A depicts a flip-chip package (100) comprising a package cover (encapsulation) (110) and a first level package carrier (115) (e.g., ceramic or plastic substrate) with an array of large solder balls (130) (e.g., BGA, ball grid array) that are used for bonding the substrate (115) to a card or printed circuit board (not shown). An IC chip (120) having a front side (145) and a back side (146) is flip-chip bonded to bond sites on the carrier (115) using an array of fine pitch solder balls (140) (e.g., C4 (Controlled Collapsed Chip Connect)), which provide electrical connections to integrated circuit interconnects, transistors, components, for example, formed on the frontside (145) of the IC chip. The carrier substrate (115) provides a space transformation between electrical connections from the chip to the printed circuit board.
With such conventional flip-chip packaging design, the IC chip (120) can be optically probed from the backside (146) without interference from the metal wiring, e.g., BEOL, carrier wiring, and solder ball connections (140). Such optical probing can be performed directly on the flip-chip packaged chip (120), if desired, by removing the encapsulation or cover (110).
With other conventional packaging designs, such as TCA (temporary chip attached) packages, IC chips designed for flip-chip BGA (ball grid array) packages can be mounted to TCA packages to allow optical testing from the backside of the IC chip. TCA packages can be used for testing a chip during the chip prototyping phase.
On the other hand, IC chips that are designed for wire-bond type packaging do not allow for optical testing from the backside. FIG. 1B schematically illustrates a conventional apparatus (150) for wire-bond chip packaging. In particular, a wire-bond chip package (150) comprises a package cover (encapsulation) (160) and a package carrier (163) (e.g., ceramic or plastic substrate) with an array of large solder balls (164) that are used for soldering the substrate (163) to a card or printed circuit board (not shown). An IC chip (165) having a front side (166) and a back side (167) is backside mounted to the carrier substrate (163) using solder or other adhesives, for example. A plurality of wire bonds (180) are formed to make electrical connections between bond pads, which are formed around the perimeter of the front side (166) surface of the chip (165), to bond pads (not shown) that are formed on the carrier substrate (163).
With this type of packaging, even with the cover (160) removed, as noted above, optical access to the integrated circuit components in the front side (166) of the chip (165) is difficult and impractical due to the BEOL metallization. Thus, IC chips designed for wire-bond type packaging are generally not suitable for optical testing. In fact, optical testing cannot be performed (or is not practically feasible) for IC chips (such as wire-bond type chips) that cannot, by design, be mounted for testing using a TCA package or which cannot be packaged (e.g. flip-chip packages) in a manner that enables free optical access to the backside substrate for optical testing.
Moreover, certain chips that can not be mounted for testing in TCA packages require custom package designs. For example, when an IC chip is taken off the production line before the final metals are formed, even if the final product will use a flip-chip package, the chip cannot be mounted in a TCA package. Moreover, optical testing is not feasible when the cost for a TCA package (which can be thousands of dollars) is too high for a given testing budget. Furthermore, when an IC chip is to be used after optical testing, most packages are designed such that the IC chip will be damaged or destroyed if the IC chip is removed from the package.